It is known that a cutting edge temperature of a cutting tool during cutting exceeds about 800.degree. C. at the maximum even under an ordinary cutting condition with a cutting rate of about 100 to 300 m/min. Further, in recent years, manufacturers who use machining operations, such as especially a car manufacturer, have increased the demand for development of a tool which can be used for cutting under a condition of a higher speed or a higher feed rate than the conventional one, such as a high speed of at least 300 m/min., for example, in order to improve productivity per unit time, in consideration of the speed of NC machine tools, to reduce the production cost and to achieve shorter working hours.
However, the cutting edge temperature of the cutting tool exceeds 1000.degree. C. in such a cutting condition, and this is an extremely severe condition for the tool material. If the cutting edge temperature is increased, the cutting edge is plastically deformed by heat, to cause regression of the cutting edge position. At a temperature exceeding 1000.degree. C., further, the base material such as cemented carbide forming the tool is oxidized and wear abruptly progresses.
In order to avoid such damage of the tool caused by cutting, tools are used that have been prepared by forming various types of hard coating layers on surfaces of hard metals by chemical vapor deposition or physical vapor deposition. Historically, a tool coated with a Ti compound first appeared, and improvement of the cutting speed was attained since the same is superior in stability under a high temperature as compared to cemented carbide. Thereafter a tool prepared by further coating a Ti compound with an Al.sub.2 O.sub.3 layer of 1 to 2 .mu.m thickness was developed to make it possible to further improve the cutting speed, and hence this forms the mainstream of the current coated cutting tool.
Al.sub.2 O.sub.3 has a small standard formation free energy, and is chemically more stable than the Ti compound. Thus, it is said that an Al.sub.2 O.sub.3 film brings a great effect for suppression of crater wear in a cutting face portion that is heated to the highest temperature in the cutting edge, and is suitable for high-speed cutting. Further it is said that propagation of cutting heat is suppressed and a hard metal material of the tool base can be kept at a low temperature since heat conductivity of Al.sub.2 O.sub.3 is small. In order to develop a tool which is capable of higher speed cutting, therefore, it is expected that the Al.sub.2 O.sub.3 layer may be further thickened.
When the Al.sub.2 O.sub.3 layer is thickened, however, hardness is reduced since bulking of crystal grains forming the coating layers progresses, and a reduction of wear resistance on the flank comes into question. It has been recognized that, if such a tool is used in practice, the dimensions of the workpiece being cut are changed by regression of the cutting edge position since the progress of wear is quick, and the life of the tool is extremely short.
On the other hand, a method of preventing bulking of crystal grains by dividing an Al.sub.2 O.sub.3 layer into plural layers is proposed in Japanese Patent Publication No. 5-49750. According to this method, the grain size of Al.sub.2 O.sub.3 can certainly be reduced and wear resistance can be improved. On the other hand, boundaries between Al.sub.2 O.sub.3 and other materials are increased, and hence separation at the interfaces easily takes place. In using such a tool for cutting with a large impact such as intermittent cutting, it has generally occurred that damage is abruptly increased due to layer separation in the flank and the cutting face, which abruptly reaches the end of or terminates the tool life.
Japanese Patent Publication No. 6-15714, on the other hand, proposes a coated sintered alloy prepared by coating with an Al.sub.2 O.sub.3 layer while dividing the same into an inner layer of 1 to 3 .mu.m thickness and an outer layer of 0.4 to 20 .mu.m thickness. Both heat insulation and wear resistance are expected as the roles of the Al.sub.2 O.sub.3 film of the outer layer. However, the function of the outer layer as an adiabatic layer is reduced by wear in an early stage, while no specific advice or consideration is given as to wear resistance of the outer layer either. Thus, progress of wear is quick, and the life of the tool was extremely short.
A technique of employing a ZrO.sub.2 film whose standard formation free energy is small similarly to Al.sub.2 O.sub.3 with smaller heat conductivity than Al.sub.2 O.sub.3 is also proposed in Japanese Patent Publication No. 52-43188 or Japanese Patent Publication No. 54-34182. However, no tool employing ZrO.sub.2 as a coating layer has been put into practice up to now. This is because a ZrO.sub.2 layer is inferior in wear resistance since the hardness of ZrO.sub.2 is low as compared with Al.sub.2 O.sub.3.
Japanese Patent Publication No. 56-52109 discloses a technique of successively coating a cutting tip of cemented carbide with three layers of a lower layer, an intermediate layer and an upper layer. The lower layer is any one of titanium carbide, titanium nitride and titanium carbo-nitride of 1.0 to 10.0 .mu.m in thickness, the intermediate layer is aluminum oxide of 0.1 to 5.0 .mu.m in thickness, and the upper layer is any one of titanium carbide, titanium nitride and titanium carbo-nitride of 0.1 to 3.0 .mu.m in thickness. This publication describes that the thickness of the intermediate layer must not exceed 5.0 .mu.m since toughness is reduced if the intermediate layer exceeds 5 .mu.m. Further, the publication describes that the thickness of the upper layer must not exceed 3.0 .mu.m since crystal grains forming the coating layers are bulked when the thickness of the upper layer exceeds 3.0 .mu.m and this is not preferable.
Japanese Patent Laying-Open No. 54-28316 also discloses a technique of forming coating layers of a three-layer structure on cemented carbide. The coating outermost layer consists of a nitride and/or a carbo-nitride of at least any one of Ti, Zr and Hf, the intermediate layer consists of Al.sub.2 O.sub.3 and/or ZrO.sub.2, and the coating innermost layer consists of a carbide and/or a carbo-nitride of at least any one of Ti, Zr and Hf. In its concrete example, the thickness of the innermost layer is 3 .mu.m, the thickness of the intermediate layer is 1 .mu.m, and the thickness of the outermost layer is 2 .mu.m. The thickness of the outermost layer is not more than the thickness of the innermost layer.
The conventional coated hard metal material having these three-layer coatings is characterized in that it has the coating of TiN or TiCN in a thickness of not more than 3 .mu.m on the oxide layer. However, when a cutting tip made of such a conventional coated hard metal material is employed in high-speed cutting, particularly in such cutting conditions in which the cutting edge temperature exceeds 800.degree. C., there have been such problems that the cutting edge of the tip is easily damaged, and dimensional change of the workpiece easily takes place. This can also be read from the description of the aforementioned publication in that the outermost layer is oxidized in high-speed/high-feed cutting and an oxide such as Al.sub.2 O.sub.3 or ZrO.sub.2 is directly exposed.